France's Chauvet Cave is guarded by a half-tonne reinforced door that seals it entrance, the code known only by three people, with cameras keeping watch 24 hours a day.
But AFP was given a rare chance to step through this gateway into prehistory and into the depths of the Grotte Chauvet in southern France -- home to the earliest known figurative drawings and now a World Heritage site.
For tens of thousands of years, time stopped in the cave nestled deep in a limestone cliff that hangs over the lush, meandering Ardeche River, until it was discovered in 1994 by a group of cave experts.
To reach the site, which is closed to the public, the lucky few allowed access must hike up a path that our Cro-Magnon ancestors once used, not far from a natural stone bridge that straddles an abandoned part of the river.
Some 36,000 years ago -- the age of the cave paintings -- tall Scots pines lorded over the cliff in a climate equivalent to that of present-day southern Norway.
After arriving at the entrance in sweltering heat, descending into the Paleolithic den brings a sharp drop in temperature and almost 100 percent humidity.
Marie Bardisa, the curator of the site, types in the code to the fortified door and it slowly swings open.
Visitors must put on white overalls and special shoes to avoid polluting the environment, as well as a helmet and harness.
"The idea is to keep the cave in the same state of containment as when it was discovered," Bardisa says.
"We watch over the atmospheric balance, we monitor the potential proliferation of algae, mushrooms or bacteria."
- Miraculously preserved -
Now begins the travel through time. After crawling through a narrow tunnel, visitors reach man-made stairs. At the bottom, the silent, cool cave opens up.
Nearly everything has been left as it was when Jean-Marie Chauvet, Christian Hillaire and Eliette Brunel stumbled across the grotto on December 18, 1994.
Crystals on huge limestone formations sparkle in the lamp light. Bones coated with clay and calcite litter the cave, proving that bears lived here before and after humans passed through. The skull of an Alpine ibex, a species of wild goat, smiles through immaculate teeth.
Visitors are not allowed to walk freely through the site but must stick to a tiny walkway that makes movement difficult.
Paintings of hands -- made using a technique of blowing red ochre pigment onto the wall around the hand -- appear out of the dark as a guide shines a powerful lamp onto the wall.
Further away, an image of a red bear with a spotty face stands over the only known drawing of a panther among all cave paintings from the Paleolithic era.
"Chauvet alone houses 75 percent of big cats and 60 percent of rhinoceroses" known to have been drawn during the period, says Charles Chauveau, the site's deputy curator.
- Remains of fires -
Even more astounding, remains of fires that look like they were only extinguished yesterday indicate where men would burn wood to make torches and charcoal.
At the far end of the large grotto, a bear's skull sits atop a stone.
Experts say Cro-Magnon man did not live in the cave but probably used it for religious reasons.
"They were hunter-gatherers and the underground world had connotations of the supernatural," said prehistorian Jean Clottes, the first expert to visit the cave 20 years ago.
All of a sudden, a group of horses painted with charcoal appears in the glare of the lamp, the artist having used a slit in the rock to represent the mouth of one of the animals.
Lions, owls, mammoths... scores of beasts reveal themselves.
The paintings are more than twice as old as those in the famed Lascaux caves also in southern France.
The cave has been miraculously preserved because it was sealed off 23,000 years ago by a rockfall. It will never be open to the public due to the fragile state of the more than 1,000 paintings.
As a result, French authorities are building a replica of the cave not far from the original which should open its doors next year.